TFC membranes are membranes that have layers of dissimilar materials joined together to form a single membrane. This layered construction permits the use of material combinations that optimize performance and durability of the membrane. TFC membranes are used for nanofiltration, and in reverse osmosis and forward osmosis membranes for treating tap water, brackish water and sea water. Such membranes typically are made by interfacial polymerization of a monomer in a nonpolar (e.g., organic) phase together with a monomer in a polar (e.g., aqueous) phase forming a discrimination layer on a porous support layer. TFC membranes are used where flux and substantial rejection characteristics are required, e.g., in the purification of water. Various materials and chemical additives have been added to TFC membranes during polymerization to increase flux without reducing rejection characteristics. TFC membranes are also subject to fouling resulting in reduced flux as contaminants, e.g., material from the brackish or sea water to be purified, build up on the surface of the TFC membrane.
When designed for reverse osmosis (RO), the TFC membrane can be made of any material, and can take any form, as long as it is capable of performing reverse osmosis, that it, it is capable, under a suitable applied pressure, of preferentially permeating water and rejecting dissolved salts. Typical reverse osmosis TFC membranes can include a felt backing layer, a support layer, and a discrimination layer. The discrimination layer can be a polyamide film that can be used to filter the liquid, usually removing salt from water. The support layer is located between the discrimination layer and the felt backing layer and generally is a porous polymer material such as polysulfone to allow the flow of filtered fluid. The felt backing layer is typically porus so as to allow fluid that has been filtered by the discrimination layer to flow through the membrane sheet.
The interfacially polymerized discrimination layer of the TFC membrane can be made by any of the known techniques. Exemplary embodiments of the discrimination layers, their chemistry and method of manufacturing them are disclosed in U.S. Pat. Nos. 4,039,440; 4,277,344; 4,902,424; 5,108,607; 5,543,046; 6,337,018; 7,109,140; and 8,177,978, each of which is herein incorporated by reference. Preferably the discrimination layer is a crosslinked polyamide. After interfacial polymerization and formation of the discrimination layer, the TFC membrane typically is dried, then washed to remove excess reactants prior to final drying or application of a frontside coating, such as a protective layer or an anti-fouling coating layer. The final drying and/or coating of the membrane can decrease the water flux through the membrane. This negative impact on water flux can be caused by the collapse of pores during the drying process.
In addition to the discrimination layer and the support layer, any number of other layers can be present on the TFC membrane, such intermediate layers between the support and discrimination layers, and coating layers (including those resulting from chemically modifying the surface of the discrimination layer) added as a protective layer, or to change the charge characteristics of the surface of the discrimination layer, or to improve the rejection properties, or to reduce fouling. Theses coating layers coat the reverse osmosis membrane on the side that will be exposed to the feed solution when the membrane is in use.
Various protective, anti-fouling and performance enhancing coating layers are known (see U.S. Pat. Nos. 6,177,011; 6,413,425; 7,490,725; 8,011,517; 8,017,050; 8,443,986; and 9,022,227, and U.S. Pat. Appl. Pub. Nos. 2010/0051538 and 2012/0111791). Application of these coatings layers can have a negative impact on water flux through the membrane, which can be caused by the drying process generally using to cross-link or dry the coating.
What is needed is a process that can minimize or eliminate the negative impact on the water flux through the membrane often observed upon drying and/or the addition of a coating layer to the TFC membrane.